Trash mobs are boring only when they're created to be boring.
WoW has had plenty of times when Trash Mobs were interesting to fight (I mean, don't look at Unsub from Azeroth or Warlords of Boredom expansions...)
I remember raids such as Blackwing Descent, where you before Magmaw, 2 big bad trash enemies that needed to be tanked, that had stacks they built up forcing tank swaps (While also keeping them apart from each other). Trash before Atramedes having a bunch of unique skills and whenever you killed one of them, each remaining enemy was fully healed and gained the skill of the killed enemy in addition to their own unique skill (With the last enemy being super OP with all the skills).
Raids like Firelands that had things like the trash before Alysrazor that had unique mechanics to content with (Making the big birbs shoot fire at the eggs/each other). Trash before Baleroc that you either needed to be able to DPS down fast, or you had to deal with the mini-boss they summoned.
Raids like Ulduar that had mini-bosses galore. Some of which became optional "Hard Modes" for certain bosses (Such as Freya being more powerful the more keepers you left up. But dropping better loot as a result)
Or even Hyjal Summit which had trash that was, in of itself, like a collection of FATEs, where you'd have to fight off waves of enemies and protect NPC's (Which, to be fair, was a little tedious but mostly because the final wave spawned the boss which meant wipes had you doing all the waves from the start again...)
Trash can be designed to be as interesting as actual boss fights. Especially when you consider that some boss fights are Council style fights which makes them essentially big Trash >.> Can easily apply the same mechanics to "Trash" mobs. While also making them worthwhile, by letting them give loot.
It's just that designers for content tend to relegate Trash to just being annoying and worthless speed humps in the path of killing bosses so put 0 effort into making them interesting or worth fighting...